The goal of the project is to develop a four-hour cooling system that replaces ice packs in mine rescue reentry rebreathers that (unlike ice) will maintain the temperature of the air returned to the miner from the CO2 absorber at a constant, safe temperature. TDA's cooling technology maintains the temperature of the return air at a constant level regardless of the level of exertion of the miner, or time-on-stream. Mine rescue rebreathers are self-contained, closed circuit, backpack sized units used by mine rescue personnel to reenter a mine after an accident to rescue stranded miners and perform other tasks necessary to bring the mine back online. Mine rescue rebreathers conserve air by recirculating exhaled air through a system that chemically scrubs out CO2 and meters in O2 (from a small compressed gas cylinder) to maintain an O2 content of at least 19.5% in the inhaled air. Considerable heat is generated when the CO2 is removed in the scrubber bed and moisture is condensed, and this heat must be removed or the air rapidly becomes too hot to breathe. In current commercial rebreathers, the heat of CO2 absorption is removed with ice packs. Commercial rebreathers are tested according to 42CFR 84.103 and must maintain the return air temperature to the miner below 35C (95F) for the entire 4-hour service time of the unit. Unfortunately, passing this test does not guarantee that this temperature will not be exceeded with different users or levels of exertion (both of which can increase the rate of metabolic CO2 generation) or different ambient air temperatures in the mine (the testing is done with an ambient air temperature of 24C). Thus, TDA Research Inc. proposes to develop a cooling system that does not use ice that provides cooling that is proportional to the heat load being generated. For example, if the miner's exertion increases, more CO2 is produced because of a higher respiration rate and this produces more heat. TDA's system responds by increasing the cooling rate. When the miner is at rest, less cooling is required and TDA's system self-adjusts for this condition. Thus, the inhaled air from the rebreather is at a constant temperature, regardless of the heat load generated by the miner. This is in contrast to ice packs, where breathing air is overcooled during times of low exertion but gets hot when the user is working hard. As a result, ice packs overcool at first, and then barely provide enough cooling when the ice has melted near the end of the four hour life. Another benefit is that TDA's design provides immediate cooling (instant on) as soon as the rebreather is activated. Finally, because no ice packs are used, the logistics for both mining and first responders are simplified.